1. Field of the Invention
The present invention relates to a method for producing an optical device having an optical element formed as a hologram on a transparent base member, to such an optical device, to an image display apparatus employing such an optical device, and to a head-mounted display employing such an image display apparatus.
2. Description of Related Art
When used embedded in a transparent base member, an optical element such as a hologram, half-mirror coat, or beam splitter layer is immune to ambient conditions such as humidity and the presence of oxygen. This makes such an optical element useful as a combiner in, for example, a head-up display or head-mounted display.
In particular, a hologram exhibits high wavelength selectivity and angle selectivity, and is therefore very useful as a combiner as mentioned above. A hologram is formed of a hologram photosensitive material, which typically is a photopolymer, silver-halide-based material, or gelatin bichromate. Among these materials, photopolymers, unlike silver-halide-based materials or gelatin bichromate, permit easy production by a dry process, and are sensitive over the entire range of visible light. Thus, photopolymers find wide application as hologram photosensitive materials.
Methods for fabricating a color hologram wherein a photopolymer is used as a hologram photosensitive material are disclosed, for example, Japanese Patent Application Laid-open No. H7-160183 (hereinafter Patent Publication 1) and Japanese Patent Application Laid-open No. 2000-276036 (hereinafter Patent Publication 2).
According to Patent Publication 1, the amounts of exposure to R, G, and B light are adjusted according to the exposure sensitivity of the hologram photosensitive material so as to obtain diffraction efficiency that produces a desired hue. According to Patent Publication 2, one or more of the peak wavelength, the peak height (diffraction efficiency), and the full-width-at-half-maximum of the diffraction efficiency of the hologram are so controlled as to prevent degraded reproduction of the white color.
When a fabricated color hologram is irradiated with light from a reproduction light source, it produces light called reproduction light. The hue (color balance) of the reproduction light of a color hologram and the brightness of the image conveyed by the reproduction light need to be adjusted with consideration given not only to the wavelength (diffraction wavelength) and the efficiency at which the color hologram diffracts light but also to the intensity of the light emitted from the reproduction light source at different wavelengths (for examples, at R, G, and B wavelengths). This is because what type of light source is used as the reproduction light source (at what intensity the reproduction light source emits light at different wavelengths) affects the hue of the production light and the brightness of the image. Accordingly, even when diffraction efficiency is so adjusted that the reproduction light is white, if no consideration is given to the intensity at which the reproduction light source emits light at different wavelengths, the reproduction light may be other than white depending on the reproduction light source actually used. With this, therefore, it cannot be said that the hue of the reproduction light is adjusted to be quite white.
From the above perspective, Patent Publications 1 and 2 mentioned above are no exception, according to which the diffraction efficiency and the amount of exposure of the hologram are adjusted with no consideration given to the intensity at which the reproduction light source emits light at different wavelengths, and thus the hue of the reproduction light and the brightness of the image vary depending on the reproduction light source actually used. That is, inconveniently, it is impossible to obtain, with every reproduction light source used, a bright image with a good color balance.